The effect of chamfer length c and nanoparticle volume fraction q on heat transfer and fluid flow is investigated numerically for water-Al2O3 nanofluid flow through various chamfered ducts of square cross section. A mixture model is used for the nanofluid flow analyses based on the finite volume method. Chamfer lengths of the ducts are assumed to be 1.0, 2.0, 3.0 and 4.0 mm; and the volume fraction of Al2O3 nanoparticles, where diameter is 10 nm, are considered as 0.01, 0.02, and 0.03. The fluid flow is examined under turbulent flow conditions (4000 Re 10,000). The results show that as the nanoparticle volume fraction increases, heat transfer and pressure drop increase in all the cases considered. Moreover, increasing the chamfer length of the duct has a positive effect on convective heat transfer, despite the negative effect on the pressure drop. The highest thermohydraulic performance is obtained as 1.118 for the case of c = 4 mm, phi = 0.03, and Re = 4000.